I have a question about Bi-polar Disorder and my girlfriend?

i know that bipolar disorder can go untreated for many years because it is hard to diagnose. but i know there is mania and depression. i know there are many other things about bipolar disorder, but my girlfriend who i have been dating for a year says she wants to go sky diving, and learn how to rollerblade, and learn how to surf, and do this and this and this, and then LITTERALLY the next day she could be crying and so depressed and feel so low about herself and things going on in her life. it switches everyday or every couple of days and its confusing and hard to keep up with. i love her so much but its tiring and i wish she wouldnt have the depressed days at all. because she is SOO much fun to be around and with and so full of life and have so much energy, then the next day she is the complete opposite. she has told me on one of her down days that nothing in life makes her truly genuinly happy anymore. she doesnt feel like herself anymore. because she used to just have the really high days and not the depressed days at all. can anyone give me advice? is this bipolar disorder? thank you